There are numerous coal deposits in the United States that are afire, many of which have been burning for 10 years or more. Some underground coal fires are burning in coal deposits that never have been mined, the fire having started at an outcrop of the coal. Other underground coal fires are burning the residual coal in abandoned mines. In both cases potentially valuable natural resources are being consumed for no useful purpose. Noxious gases are generated without control from such fires, resulting in a hazardous environment in the vicinity of each fire. Further, the fires consume large quantities of coal and thus create void space underground which in time will result in subsidence of the overburden. Such subsidence often opens cracks from the surface of the ground into the underground coal. These cracks can serve as conduits to supply fresh air to the fire or to serve as chimneys to remove the products of combustion from the fire.
In the presence of an oxidizer such as air, coal will burst into flames when its temperature reaches the ignition point of approximately 800.degree. F. The sequence of events leading to fire need not begin, however, with a temperature nearly so high. Once coal warms to a temperature near that of the boiling point of water (212.degree. F), if a supply of air is present, the sequence will continue to ignition by so-called spontaneous combustion when the heat of oxidation builds up more rapidly than it dissipates. Other unplanned ignition of a coal deposit can occur as a result of grass or brush fires, warming fires kindled by persons outdoors on a cold day, lightning striking the earth and the like.
When an underground coal deposit is ignited it will burn to resource exhaustion if oxygen remains available. In many cases the fire may propagate undetected for weeks or months and in some cases years. The longer a fire proceeds the more difficult its eventual control is apt to be. The unplanned fire underground propagates with the same combustion chemistry as the planned and controlled fire above ground. With an abundant supply of oxygen, carbon in the coal burns to carbon dioxide, hydrogen burns to water vapor and sulfur burns to sulfur dioxide. Most unplanned fires underground, however, propagate with less than an abundant supply of oxygen resulting in much of the carbon converting into carbon monoxide, some of the hydrogen combining with sulfur to form hydrogen sulfide and some of the hydrogen combining with oxygen to form water vapor. With carbon monoxide and hydrogen sulfide emanating from the underground fire, the hazards to people and animals nearby is significantly increased.
When the unplanned underground fire has been underway for a considerable period of time, substantial portions of the underground area have been subjected to high oxidation temperatures in the order of 2,000.degree. F and the carbon in the coal is incandescent. In underground areas where there is room for flames, temperatures in the order of 3,000.degree. F are not uncommon. Attempts to douse the fire with water are generally unrewarding because the water reacts with incandescent coal to form hydrogen and carbon monoxide, both of which are potent fuels that may serve to intensify the fire.
One simple way of stopping the fire is to cut off the oxygen supply, for example by piling dirt over each conduit to the fire. Such a procedure will stop the oxidation underground, but does little to dissipate the heat in the former burn area underground. The former burn area underground will remain at a temperature above the ignition temperature of coal for long periods of time, in some cases a decade or more. Should the underground hotspot gain a new source of oxygen, such as rain washing away the dirt cover or new cracks forming from subsidence, the fire will rekindle.
Other methods of stopping the fire include grubbing out the fire area in its entirety, grubbing out the coal ahead of the fire front and allowing the fire to burn itself out, flushing the fire area with an inert material such as fly ash, and the like. All such methods involve costs without offsetting revenues. Most methods require the test of time to assure that the method has been successful.
All methods of controlling underground coal fires require a certain amount of disruption of normal activities conducted at the surface of the ground. If the fire is located in a populated area, some of the population may be required to relocate either temporarily or permanently. Such disruption adds to the costs of controlling the fire. A considerable improvement over present methods can be attained by converting the products of combustion to commercial products, thereby eliminating the hazards of migrant noxious gases and generating revenues to offset costs. It is an objective of the instant invention to teach such methods.